Khẳng định nào sau đây là sai?

\({\sin ^2}\alpha + {\cos ^2}\alpha = 1\).

$1+co{t}^2\alpha =\frac{1}{{\sin }^2\alpha }$

\(\tan \alpha .\cot \alpha = - 1{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} \left( {\sin \alpha .\cos \alpha \ne 0} \right)\).

\(1 + {\tan ^2}\alpha = \frac{1}{{{{\cos }^2}\alpha }}{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} \left( {\cos \alpha \ne 0} \right)\).